1. Field of the Invention
The present invention relates to a crosslinkable composition cross-linkable by real Michael addition (RMA) reaction and resins for use in said composition. Real Michael Addition is a reaction wherein a reactive component B with at least 2 activated unsaturated groups (hereafter also referred to as the RMA acceptor) and a reactive component A with at least 2 acidic protons C—H in activated methylene or methine groups (hereafter also referred to as the RMA donor) react in the presence of a strong base catalyst.
2. Description of the Related Art
RMA chemistry can be tuned to give very fast curing compositions (also at lower curing temperatures) in coating compositions at acceptable or good pot lives and good material properties, which makes this chemistry very attractive as a basis for coating compositions. Details of RMA cross-linkable compositions using a latent based cross-linking catalyst are described by inventors of the present application in WO2011/055463 which is herewith incorporated by reference.
Real Michael addition is activated by strong bases. In tuning the reactivity of coating systems in view of achieving a desirable drying profile, there are various requirements to balance. The drying profile (also referred to as the reaction profile or as the curing profile) is the progress of the cross-linking reaction as a function of time. It is required that the drying profile allows build-up of mechanical properties as fast as possible, to help the productivity of the coater. There is also a desire for crosslinkable compositions that can be simply cured in ambient conditions, as opposed to for example compositions comprising photo-latent amine catalysts, known from T. Jung et al Farbe and Lacke October 2003.
On the other hand, it is required to have a good appearance of the resulting coating. This implies the need for sufficient levelling during the immediate period after application, when the curing coating composition is present as a liquid and capable of such levelling. This also implies the need for absence of artefacts like solvent inclusions or gas inclusions or other surface irregularities that may occur if curing is very fast, especially if it is faster at the surface than in deeper layers, which is often the case if curing occurs at the time scale of solvent evaporation or surface activation of a catalyst. Also, film hardness build-up will be affected under conditions in which solvent entrapment occurs. The described requirements are to some extent opposing each other. For a fast curing profile high levels of catalyst are preferred, whereas at the same time such high levels of catalysts may negatively influence surface appearance and hardness development.